(1) Field of the Invention
The present invention relates to a rubber composition having an improved cold resistance, and comprising, as a rubber component, a nitrile group-containing copolymer having a specific composition and a reduced amount of an unsaturated bond.
(2) Description of the Related Art
When used for a control of automobile exhaust emissions, rubber parts disposed in the vicinity of an engine must have a high heat resistance. To meet this demand, a nitrile group-containing, highly saturated copolymer rubber formed by hydrogenating a part or all of carbon-to-carbon double bond-containing monomer units in a nitrile group-containing hydrocarbon rubber such as an acrylonitrile/butadiene copolymer rubber (hereinafter referred to as "NBR") was developed and is now in use (see U.S. Pat. No. 4,404,329).
This rubber has an ozone resistance and heat resistance superior to those of conventional NBR, but has a cold resistance inferior to that of NBR in some cases depending upon the nitrile group content or the unsaturation degree of the main chain of the polymer.
Due to recent advances in this field, the range of temperatures of the atmospheres in which various rubber parts are used has been extended to lower and higher temperatures and therefore, the development of a nitrile group-containing, highly saturated copolymer rubber composition having an improved cold resistance is urgently required in this field.
In a conventional NBR, a reduction of the nitrile group content will improve the cold resistance. For example, in the TR test specified in ASTM D-1329 (an elongated test piece is frozen, the temperature is gradually elevated, and the recovery of the elongated test piece is measured: more specifically, the temperature at which the length of the test piece is contracted, i.e., recovered, by 10% by elevation of the temperature is designated as TR10), NBR having a bonded acrylonitrile content of 37% by weight shows a TR10 value of -20.5.degree. C., but if the bonded acrylonitrile content is reduced to 28% by weight, the TR10 value is lowered to -31.degree. C.
Nevertheless, a nitrile group-containing, highly saturated copolymer rubber is different from NBR in that, even if the nitrile group content is reduced, the cold resistance is not always improved. For example, the TR10 of a rubber comprising 5% by weight of butadiene units and 37% by weight of acrylonitrile units, with the balance being hydrogenated butadiene units, is -24.5.degree. C. but the TR10 of a rubber comprising 5% by weight of butadiene units and 28% by weight of acrylonitrile units, with the balance being hydrogenated butadiene units, is -20.degree. C., and the cold resistance is not improved even if the acrylonitrile content is reduced.